Mid 2011 Macbook air will not mirror

I have a mid 2011 macbook air and a 2nd generation apple tv. All software is up to date. I can use air play in i-tunes on the macbook air, but there is no screen mirroring option in the main menu bar. Apple documentation says this model supports mirroring,(Intel Core i5) but the best I can get is "detect displays" on the top menu bar. Am I missing something?

(i-pad mirrors to ATV just fine)